A solid polymer type fuel cell is a fuel cell using a solid polymer such as ion-exchange resin as an electrolyte, and is relatively low in operation temperature. The solid polymer type fuel cell has, as shown in FIG. 1, a basic structure wherein a space surrounded by cell bulkhead 1 having a fuel flow hole 2 and oxidizing agent gas flow hole 3, respectively communicated with outside, is divided by a membrane assembly in which a fuel chamber side catalyst electrode layer 4 and an oxidizing agent chamber side catalyst electrode layer 5 are bonded to both surfaces of a solid polymer electrolyte membrane 6 respectively, to form a fuel chamber 7 communicated with outside via the fuel flow hole 2 and an oxidizing agent chamber 8 communicated with outside via the oxidizing agent gas flow hole 3. Then, in the solid polymer type fuel cell having the above basic structure, a fuel such as hydrogen gas or liquid fuel including methanol, etc. is supplied into said fuel chamber 7 via the fuel flow hole 2, and oxygen or oxygen containing gas such as air to act as an oxidizing agent is also supplied into the oxidizing agent chamber 8 via the oxidizing agent gas flow hole 3. Furthermore, an external load circuit is connected between both catalyst electrode layers to generate electric energy by the following mechanism.
When using a cation-exchange membrane as the solid polymer electrolyte membrane 6, a proton (hydrogen ion) generated by contacting a fuel with a catalyst included in the electrode in the fuel chamber side catalyst electrode layer 4 conducts in the solid polymer electrolyte membrane 6 and moves into the oxidizing agent chamber 8 to generate water by reacting with oxygen in the oxidizing agent gas in the oxidizing agent chamber side catalyst electrode layer 5. On the other hand, an electron, generated in the fuel chamber side catalyst electrode layer 4 simultaneously with the proton, moves to the oxidizing agent chamber side catalyst electrode layer 5 through the external load circuit, so that it is possible to use the above reaction energy as an electric energy.
As a cation-exchange membrane used in the fuel cell as the solid polymer electrolyte membrane, a perfluorocarbon sulfonic acid resin membrane is most commonly used. However, in the fuel cell using the perfluorocarbon sulfonic acid resin membrane, there are big problems such that only noble metal catalyst is usable due to the strongly acidic reaction field, and that the perfluorocarbon sulfonic acid resin membrane is also expensive, resulting in limitations in cost reduction.
To solve the above-mentioned problems, it has been examined to use an anion-exchange membrane instead of perfluorocarbon sulfonic acid resin membrane, and several of such solid polymer type fuel cells have been already proposed (Patent Articles 1 to 4). Since the reaction field is basic in the fuel cell using the anion-exchange membrane, catalysts other than noble metal catalyst can be used. In this case, the mechanism for generating electric energy in the solid polymer type fuel cell is different in ion species moving through the inside of the solid polymer electrolyte membrane 6. Namely, hydrogen gas or methanol and the like is supplied to the fuel chamber side, and oxygen and water are supplied to the oxidizing agent chamber side, so that the catalyst included in the electrode is contacted with the supplied oxygen and water to generate a hydroxy-ion in the oxidizing agent chamber side catalyst electrode layer 5. The hydroxy-ion conducts in the above solid polymer electrolyte membrane 6 comprised of a hydrocarbon-based anion-exchange membrane and moves to the fuel chamber 7 to generate water by reacting with fuel in the fuel chamber side catalyst electrode layer 4. Along with the above, an electron generated in the fuel chamber side catalyst electrode layer 4 is moved via the external load circuit into the oxidizing agent chamber side catalyst electrode layer 5, and the reaction energy is used as electric energy.
Also, in the solid polymer type fuel cell using the anion-exchange membrane, it is expected to notably inhibit the phenomenon to permeate methanol and the like as fuel from the fuel chamber side to the oxidizing agent chamber side, i.e. crossover, which is a problem with the use of the cation-exchange membrane, especially the above-mentioned perfluorocarbon sulfonic acid resin membrane. Because of the difference in atmosphere of both electrodes and the expanded scope of selection of usable catalyst types, it is further expected to improve electric voltage due to lowered overvoltage of oxygen reduction, availability of fuel having carbon-carbon bond, the selection of a catalyst for the oxidizing agent side inactive for the crossovered fuel, etc.
As the anion-exchange membrane 6 used in the solid polymer type fuel cell using an anion-exchange membrane, in general, there can be used a membrane having a copolymer of a monomer such as styrene, vinyl toluene, ethyl styrene, vinylpyridine and vinylpyrazine and a crosslinkable monomer such as divinylbenzene, trivinylbenzene, divinyl toluene and divinyl xylene as its base, and normally including a quaternary ammonium group, quaternary phosphonium group and the like as its anion-exchange group. The anion-exchange membrane is normally produced by mixing a monomer having a functional group convertible to the above quaternary ammonium group or quaternary phosphonium group with other monomer, followed by polymerization to form a membrane-like material, and then converting the functional group to a quaternary onium group, and there may be mentioned the following production methods a) to c), for example.
a) a method in which a raw membrane having a halogenoalkyl group for forming an anion-exchange membrane is reacted with an agent having a tertiary amino group for forming quaternary ammonium or an agent having a tertiary phosphine group for forming quaternary phosphonium quaternary phosphonium;
b) a method in which a raw membrane having a tertiary amino group or tertiary phosphine group for forming an anion-exchange membrane is reacted with an alkylating agent having a halogenoalkyl group; and
c) a method in which a polymerizable composition including a polymerizable monomer containing a quaternary ammonium group or quaternary phosphonium group having a halogeno ion as its counterion is polymerized.
In the production methods, the counterion of the above quaternary ammonium group and quaternary phosphonium group is normally a halogeno ion such as chloride ion and bromide ion in the obtained anion-exchange membrane.
When an anion-exchange membrane having a halogeno ion, such as chloride ion, as its counterion (hereinafter, this anion-exchange membrane may also be referred to as “halogen type ion-exchange membrane”) is used as the solid polymer electrolyte membrane of a fuel cell, catalyst poisoning due to the halogeno ion can be concerned. Furthermore, in the resulting solid polymer type fuel cell, the halogeno ion competes in conduction against the hydroxy-ion in the anion-exchange membrane to increase the internal resistance of the cell and to decrease the concentration of the hydroxy-ion as an electrode reacting species, so that the fuel cell output is declined. In these conditions, it is highly preferable to preliminarily ion-exchange the counterion of the anion-exchange membrane from the halogeno ion to the above hydroxy-ion (hereinafter this anion-exchange membrane may also be referred to as “OH type anion-exchange membrane”) to eliminate such catalyst poisoning and further to increase the concentration of the hydroxy-ion as a conducting species as well as the electrode reacting species. As the method, there may be mentioned a method in which the halogen type anion-exchange membrane is produced followed by impregnation of the same with a solution of sodium hydroxide (NaOH).
However, sodium hydroxide (NaOH) is a strongly basic deleterious substance, and ion-exchange operation with the above solution of sodium hydroxide requires careful handling in view of safety and the like.
Also, repeated impregnation is required with exchanging the sodium hydroxide solution to a new one because the ion-exchange from the halogen ion to the hydroxy-ion via impregnation with the sodium hydroxide solution is inefficient and only one impregnation is insufficient. Consequently, the ion-exchange operation requires long time, and complicated operations such as exchange of the sodium hydroxide solution.
Furthermore, the OH type anion-exchange membrane produced with strongly basic solution is slightly lowered in ion-exchange capacity compared to the original halogen type anion-exchange membrane, which makes it difficult to produce an anion-exchange membrane having stable ion conductivity. This may be because a part of quaternary ammonium groups is degraded due to the severe condition at the time of the above impregnation with the sodium hydroxide solution (Patent Document 5).
[Patent Document 1] The Japanese Unexamined Patent Publication H11-135137
[Patent Document 2] The Japanese Unexamined Patent Publication H11-273695
[Patent Document 3] The Japanese Unexamined Patent Publication 2000-331693
[Patent Document 4] The Japanese Unexamined Patent Publication 2002-367626
[Patent Document 5] The Japanese Unexamined Patent Publication 2002-105138
As stated above, when an anion-exchange membrane is used as a solid polymer electrolyte in a solid polymer electrolyte type fuel cell, an OR type counterion, instead of a halogen type counterion, can advantageously be used as a counterion of the anion-exchange membrane, but the ion-exchange operation has not been well developed. Therefore, it has been a big problem to develop a method for producing such an anion-exchange membrane for a solid polymer electrolyte type fuel cell, safe and easy in operation, and able to highly inhibit degrading a quaternary ammonium base to obtain high battery output.